Continuous regeneration of ferric sulfate pickling bath

ABSTRACT

An electrolytic process for regenerating a ferric sulfate pickling liquor by continually circulating the liquor from an operating pickling tank through an electrolytic cell to oxidize some of the ferrous ions to ferric ions. The cell is operated under conditions such that all regenerated products can be returned to the pickling tank.

United States Patent Stanley J. Beyer Louisville, Ky.

Nov. 14, 1960 Nov. 23, 1971 General Electric Company inventor Appl. No. Filed Patented Assignee CONTINUOUS REGENERATION 0F FERRlC SULFATE PICKLING BATH 18 Claims, 7 Drawing Figs.

US. Cl .I 204/130, 204/93, 204/145 R, 204/237 Int. Cl BOlk 1/00, C23b 1/04, BOlk 3/00 Field of Search 204/145, 130, 232, 237, H2

[56] References Cited UNITED STATES PATENTS 788,064 4/ l 905 Ramage 204/130 l,954,664 4/1934 Cain 204/130 2,389,691 1 1/1945 Schumacher et al. 204/l 12 2,583,098 l/l952 Heise et al. 204/l 12 2,810,686 l0/l 957 Bodamer et al. 204/130 3,252,879 5/l966 Sommer et al.. 204/1 12 3,425,920 2/ l 969 Frantzis 204/145 Primary Examiner-John H. Mack Assistant Examiner-T. Tufariello Attorneys-Walter E. Rule, Harry F. Manbeck, Jr., Frank L.

Neuhauser, Oscar B. Waddell and Joseph B. Forman ABSTRACT: An electrolytic process for regenerating a ferric sulfate pickling liquor by continually circulating the liquor from an operating pickling tank through an electrolytic cell to oxidize some of the ferrous ions to ferric ions. The cell is operated under conditions such that all regenerated products can be returned to the pickling tank.

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CATHODE TO REFERENCE ANODEZ TO REFERENCE I l l l I l 3 35 4- 4.5 S '55 AMPERES INVENTOR. STANLEY 3'. BEYER HIS ATTORNEY CONTINUOUS REGENERATION OF FERRIC SULFATE PICKLING BATH BACKGROUND OF THE INVENTION A well-known pickling bath used for pickling or etching iron and steel surfaces in preparation, for example, for the subsequent application of porcelain enamel consists of an iron sulfate solution adjusted to the desired pH by the addition of sulfuric acid and to the desired ferric ion content by the addition of hydrogen peroxide. Such a bath described, for example in U.S. Pat. No. 3,078,180 Lauder et al. issued Feb. 19, 1963, contains ferrous, ferric, sulfate and hydrogen ions.

The use of ferric sulfate pickling baths, in which most of the pickling is accomplished by the ferric ions, has a number of advantages over the older sulfuric acid baths in which all of the pickling action is accomplished by hydrogen ions. The ferric ion pickling is faster so that more metal can be removed from the iron surface in a given length of time. In addition, the composition of the iron alloy, i.e., the steel, does not have as great an effect on the pickling rate so that the same bath can be used to pickle steels of somewhat different compositions.

During use of a ferric sulfate bath, iron from the pickled surfaces dissolves in the bath so that the total iron content of the bath tends to increase and at the same time ferric ions are reduced to ferrous ions. The total iron content reaches an equilibrium due to the combination of (1) drag out of the pickling liquor, that is the removal of some of the solution on the surfaces of the pickled articles as they are removed from the bath, and (2) the periodic addition to the bath of makeup water and of sulfuric acid to maintain the desired pH.

In order to maintain the ferric ion content at the desired level, the ferrous ion must be oxidized to ferric ion. In present commercial operations of this type, this oxidation is accomplished by the addition of hydrogen peroxide or some other chemical oxidant at a rather substantial operating cost.

It has been proposed to regenerate ferric pickling liquors by the electrolytic treatment thereof. In most of the prior art electrolytic processes of which applicant is aware, the regeneration treatment has been concerned with the regeneration of the older sulfuric acid baths, that is baths which do not purposely contain ferric ions. Accordingly, these electrolytic regeneration processes have involved a plating out of the iron content of the spent liquor for recovery of the acid component thereof.

An early patent disclosing such a electrolytic treatment is U.S. Pat. No. 799,064 Ramage issued Apr. 25, 1905. The process of this patent involved the initial addition of sulfur dioxide to the pickling liquor followed by electrolysis to remove most of the iron as iron plating on the cathode and the utilization of the remaining liquid in the pickling process. Patents issued subsequent to the Ramage patent have, with few exceptions, been directed to the recovery of the components of a waste pickle liquor rather than to the regeneration of the liquor for reuse in a pickling bath. Also each of these patents has featured an electrolytic cell in which a membrane or diaphragm or a porous anode divided the cell into anode and cathode compartments, and only the liquid from the anode compartment was used, if used at all, in a pickling operation. Divided cells for electrolytically treating waste iron sulfate pickling liquors are disclosed for example in U.S. Pat. No. 1,006,836 Farnham issued Oct. 24, 1911; U.S. Pat. No. 1,954,664 Cain issued Apr. 10, 1934; U.S. Pat. No. 2,583,098 Heise et al. issued Jan. 22, 1952; U.S. Pat. No. 2,810,686 Bodamer issued Oct. 22, 1957 and U.S. Pat. No. 3,111,468 Kerti issued Nov. 19, 1963. Like the Ramage process, all of the processes had as their prime objectives the plating of iron on the cathode with the recovery of metallic iron therefrom and the enrichment of the remaining solution or a solution component in sulfuric acid.

The regeneration of ferric pickle liquors by the electrolytic treatment thereof has also been proposed making use of an electrodialytic pennselective membrane to divide the cell into anode and cathode compartments. Such a cell is described in the Proceedings of the Porcelain Enamel Institute Forum, Volume 26, 1964, pages 91-97, by William H. Parker. The membrane in the divided cell has a fixed positive charge used to block the transfer of positive (iron) ions to the cathode where the reduction of ferric ions would take place. At the anode, oxidation of ferrous ions readily takes place while hydrogen, supplied by the addition of sulfuric acid, is reduced at the cathode. There has apparently been no commercial use of this process, probably because of the high cost of the membrane and the need for its regular replacement.

The present invention is directed to a novel process of electrolytically regenerating a ferric sulfate pickle liquor which is substantially cheaper than the aforementioned chemical and electrochemical means for regenerating such liquors and is primarily distinguished by the employment of a cell in which the anode surface area is substantially greater than the cathode surface area. It is also distinguished from the aforementioned electrolytic processes in that (1) no diapragm or other separating means is employed in the electrolytic cell, and (2) iron is not permanently removed or plated out of the liquor during the regeneration thereof, or in other words all of the treated liquor and the components thereof, except for small amounts of hydrogen gas liberated in the cell, are returned to the pickling tank with substantial enrichment of the important ferric ion.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided an electrolytic process for continuously regenerating a ferric sulfate pickling liquor which comprises the oxidation of ferrous iron to ferric iron electrolytically by circulating the pickling liquor through an electrolytic cell wherein the conditions at the anode are favorable to the oxidation of ferrous ions to ferric ions without liberation of oxygen and the conditions at the cathode are unfavorable for reduction of ferric ion to ferrous ion and favorable for the deposition of hydrogen, or small amounts of metallic iron and hydrogen; all of these reactions being accomplished without the sue of a diaphragm membrane, or other separator for separating the cell into anolyte and catholyte compartments.

More specifically there is provided an electrolytic regeneration process which comprises continually circulating pickling liquor from a ferric sulfate pickling bath through an undivided electrolytic cell having an anode-to-cathode surface area ratio from about 6:1 to 22:1 and containing a plurality of parallel graphite anodes spaced apart a distance of from about 1.5 to 10 centimeters and wire cathodes positioned between the adjacent cathodes. A direct current is passed through the cell at an anode current density of at least 0.5 ampere per square decimeter but not exceeding that at which polarization leading to oxygen discharge takes place at the anode. The entire regeneration solution enriched in ferric ion and including small amounts of iron periodically redissolved therein from the cathode, is returned to the pickling tank and the only additions to the pickling bath are makeup quantities of sulfuric acid and water.

BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawing:

FIG. 1 is a flow diagram illustrating the process of the present invention;

FIG. 2 is an end view, in section, of a preferred electrolytic cell construction for use in practicing the present invention;

FIG. 3 is a side elevational view, in section, of the cell of FIG. 2;

FIG. 4 is a voltage-current curve illustrating the changing electrochemical reactions at the electrodes;

FIG. 5 is a curve illustrating the current efficiency of an electrolytic cell operated in accordance with the present invention;

FIG. 6 plots cell voltage-current curves for (l) anode-tocathode voltage (2) anode to reference voltage and (3) cathode to reference voltage; and

FIG. 7 is a curve illustrating the effect of flow rate on anode polarization.

DESCRIPTION OF THE PREFERRED EMBODIMENT The manner in which the pickling and electrolytic regeneration process of the present invention may be carried out is illustrated in FIG. I of the drawing. Liquor from a ferric sulfate pickling tank 1 is continually circulated by means of a pump 2 through an electrolytic cell 3 and returned through a conduit 4 to the tank 1. Tank 1 contains a ferric sulfate pickling liquor initially prepared by dissolving ferrous sulfate in water, oxidizing some of the ferrous ion to ferric ion and adjusting the pH of the solution by the addition of sulfuric acid. Such a pickling bath is employed extensively for the rapid etching of iron and steel as for example prior to the application of porcelain enamel thereto. During operation of such a bath, the iron is etched primarily by action of the ferric sulfate present in the bath and to a lesser extent by the sulfuric acid. A product of the chemical etching reactions is ferrous sulfate. To continue the efiective operation of the bath, the ferrous iron is reoxidized to ferric iron.

This oxidation of the ferrous iron to ferric iron or more specifically the continual regeneration of the pickling liquor is accomplished electrolytically in accordance with the present invention by means of the electrolytic cell 3 shown in greater detain in FIGS. 2 and 3 of the drawing.

The electrolytic cell comprises a conventional acid-resistant container 6 made of a material such as plastic lined steel, acid brick, hard rubber, methylmethacrylate, polyester, other plastic material or glass into which pickling liquor from the tank 1 is introduced through a header 7 positioned at one end of the tank and extending substantially the full width thereof; this header preferably including a plurality of spaced outlets 8. The cell 3 contains a plurality of parallel graphite plate anodes l immersed in the liquor or electrolyte l 1 being regenerated and a plurality of cathodes l2 interspaced between the anodes 10. In the illustrated embodiment of the invention, each of the cathodes 12 comprises a plurality of parallel wires 13 which are electrically interconnected and which are supported on a plastic-coated metal frame 14. The regenerated electrolyte overflows a weir 15 at the opposite end of the tank from the inlet header 7 and then passes through the conduit 4 back to the pickling tank. The cell 6 should be provided with a suitable hood for removing any gaseous products and entrained liquor released during the electrolytic regeneration of the pickling liquor.

The cathode wires 13 may be of any suitable electrically conducting chemically resistant material such as special chemically resistant stainless steels, zirconium, tantalum, gold, etc. tantalum being a preferred material.

Some of the operating conditions in the cell 3 will be dependent upon the desired operating conditions of the pickling tank I. For example, while temperature appears to have a negligible effect on the operation of the cell, the cell will generally operate approximately the same temperature as the pickling tank, that is within a range of from 50 to 80 C. and preferably about 65 C.

The pH of the solution flowing through the cell is also dependent on the preferred pickling operation conditions. The mechanism of acid ferric pickling is essentially about 80 percent ferric attack and percent acid attack throughout the normal acidities used in ferric pickling, that is within the pH range of from about 0.5 to 1.5, so that the cell will generally be operated within this range of hydrogen ion concentrations.

A lower pH favors cell current efiiciency in a minor way 7 within the limits suitable for acid ferric pickling. The most important effect of pH with regard to the electrolytic cell is the effect the acid has on the amount of iron deposited on the cathode. The more acid the solution, the less iron deposited on the cathode. Deposition of iron on the cathode does not directly affect the current efficiency for ferrous ion oxidation, because ferrous oxidation is an anodic process, but metallic iron on the cathode must eventually be removed from the cathode or the wire diameter will increase greatly and for this reason reduce the anode-to-cathode ratio and the cell efficiency.

The removal of metallic iron from the cathode is effected by periodically discontinuing the passage of direct current through the cell and allowing the iron to dissolve chemically. Such dissolution uses only a small fraction of the ferric ion generated, and therefore affects the overall ferric generation efficiency only to a small degree. In commercial installations, the time when the current is turned off can be planned for workers break" time, lunch time, overnight, or the current can automatically be turned on and off during regular operations at more frequent intervals. The electrolytic cell is preferably designed with sufficient capacity to allow off time for dissolution of cathode iron. Thus the manner of operation of the cell determines to some extent what pH limits are allowable. For around-the-clock operations where no nonoperating time is available, a pH of 0.5 or less is recommended. A pH to 1.5 may be permissible if there is nonoperating time available to allow the cathodic iron to dissolve. Broader pH limits will still result in ferrous oxidation at highcurrent efficiency. Taking all of these factors into consideration, the cell may be operated within a pH range of from 0.25 to 1.5, preferably from 0.4 to 0.7.

The ferrous ion concentration of the electrolyte as treated in the cell also depends on the desired operating conditions within the pickling bath. The majority of the pickling baths of this type operate with a ferrous ion concentration of from about 10 to 40 grams per liter and the maximum ferrous ion concentration is usually limited by solution drag out" and the addition of makeup water and acid, that is by the constant removal of pickling solution from the bath on the surfaces of the pickled articles and the periodic adjustment of the pH and volume losses by the respective additions of sulfuric acid and water.

Since the process of the present invention contemplates the continual regeneration of the pickling liquor, that is a regeneration before the ferric ion concentration falls below an operating minimum, the liquor or electrolyte circulated through the cell will contain ferric ions and the concentration of these ions is dictated by the conditions desired in the etching process. For the usual enameling iron pickling, the ferric ion concentration of the pickling liquor introduced into the cell will range from 1 to 10 grams per liter, generally between 4 and 6 grams per liter and preferably about 4 grams per liter. The ferric ion concentration of the solution of course increases as it passes through the cell while the ferrous ion concentration correspondingly decreases.

With the temperature, composition and pH of the solution entering the cell largely determined by the pickling tank operating conditions, the cell 3 is so designed and operated as to provide the desired regeneration reactions in that solution, that is an oxidation of ferrous to ferric ions with minimum deposition of iron on the cathode.

Considering first the cell design, narrow electrode spacing reduces power costs and overall cell size. However there must be room between adjacent anodes for a cathode and for solution flow. In general the graphite anodes are spaced from 1.5 to 10 cm. apart, preferably from 2.5 to 6 cm. apart. The anodes and cathodes should face each other to obtain uniform flow of current through all the electrode areas. The direction of solution flow relative to the electrodes is immaterial so long as there is relative motion between the solution and the electrode surfaces with a flow through" of the solution in order to continuously return the regenerated products to the pickling tank.

By changing the size of the cathode wires or the spacing between them or by changing the distances between the electrodes, the current efficiency characteristics can be varied to provide broader or narrower limits for the total cell current at which up to percent efficiency in the oxidation of ferrous ion to ferric ion can be obtained. However the cathode wire size should not be so small that the wires are fragile or incapable of carrying the required current without overheating. On the other hand too wide a spacing between cathode wires relative to the anode-to-cathode distance can provide local areas of high-current density on the anode plates thus resulting in a high current density and the liberation of oxygen on such local areas, a lower cell efficiency, and a slow disintegration of graphite anodes.

The anode-to-cathode surface area ratio is selected to obtain the desired conversion of ferrous ion to ferric ion at the anode and not obtain the reverse reaction at the cathode in substantial amounts. This requires that the cell be so designed that the anode current density is low and the cathode current density is high. With anode-to-cathode surface area ratios of 6:1 or higher, high-current efficiencies for oxidation of ferrous ions can be obtained. The current efficiency drops rapidly as the area ratio approaches l:l. Ratios of up to 18:1 are acceptable for the usual cathode materials. As the anode-tocathode area ratio becomes higher than 18:1, the cathode wire must be more conductive and the selection of materials which are chemically resistant and electrically conductive narrows from the cheaper materials to the more expensive materials such as tantalum or gold. Usually, the anode-to-cathode surface area ratios should be within the range of 18:1 to 18:1 and preferably about :1.

The cell is operated at an anode current density providing the desired oxidation of ferrous ions, and undesired reactions at the cathode are controlled by the anode-to-cathode surface area ratio. The maximum anode current density is that current density at which polarization leading to oxygen discharge at the anode is indicated. However this point of initial polarization is affected by the rate of flow of electrolyte passing the anode and the amount of ferrous ion present in the electrolyte.

The prime objective is to maintain conditions at the anode such that all of the electrical charge at the anode is effective to convert ferrous to ferric ions. To this end, it is desirable to operate the cell at an anode current density just below that which may lead to oxygen discharge for the ferrous ion concentration and flow rate of the electrolyte being regenerated. For example, it has been found that with about 20 grams per liter of ferrous ion present in the electrolyte and a flow of 7.5 cm. per minute past the anode, the maximum recommended anode current density is about 2.0 amperes per square decimeter. If the flow of the same solution is increased to 30 cm. per minute, the maximum anode current density is 2.5 amperes per square decimeter.- ln treating an electrolyte of twice the ferrous ion content, that is 40 grams per liter, at a flow of 7.5 cm. per minute, the initial anode polarization occurs at about 3.0 amperes per square decimeter and with the higher flow rate of 30 cm. per minute for the same solution, the point of initial polarization occurs at about 4 amperes per square decimeter.

In general, the anode current density should not exceed about 5 amperes per square decimeter and is preferably maintained between 2 and 3 amperes per square decimeter but above 1.5 amperes per square decimeter. The maximum flow rate of the electrolyte depends upon the capacity of the pump and the volume of the cell. The solution flow rate past the anode should be as high as possible but for practical reasons the flow rate will ordinarily be between 3 and 120 centimeters per minute and generally between 7.5 and 50 centimeters per minute.

The following examples serve to further illustrate the process of this invention:

EXAMPLE I The electrolytic cell was a laboratory scale cell consisting of four parallel graphite plate anodes, three-sixteenths inch thick X 2% inches wide immersed in the electrolyte to a depth of 2 /2 inches and spaced three-fourths inch apart. lnterspaced between the anodes were three cathodes, each consisting of three 0.0315 inch diameter nickel, chromium, molybdenum, copper alloy stainless steel wires, also immersed to a depth of 2% inches. The anode-to-cathode area ratio was therefore about 18:1, so that the cathode current density was always 18 times the anode current density. The pickling liquor circulated through the cell contained about 22 grams per liter ferrous ion and 4 grams per liter of ferric ion in u sulfuric acid solution of 0.1 Normal concentration. The flow rate unless otherwise indicated was 8.5 cm. per minute.

FIG. 4 is a curve of the voltage-current characteristic of this cell. These data show two significant changes in potential: l From 0.0 v., 0.0 ampere to about 0.8 v., 0.3 ampere, there is a constant slope, which then decreases at a varying rate until 1 .2 v., 1.0 a. is reached. (2) From 1.2 v., 1.0 a. to about 2.1 v., 5 a., there is a second region of constant slope before a second change in slope occurs. After the polarization is complete between 2.1 v., 5.0 a. and 3.5 v., 10.0 a., a new characteristic slope continues unchanged to very high currents.

A change in slope of the voltage-current curve indicates a change in the electrochemical reactions occurring at either the anode or the cathode. Considering all of the ions in the solution, there are three possible reactions at the cathode. These reactions with the standard potentials taken from the electromotive series chart are:

(1) Fe+++ (e) Fa 0.77 volt Since the reduction of ferric ion to ferrous ion proceeds most readily, it is this reaction which first occurs at the cathode at the lowest current density (cell current).

At the anode, there are two possible reactions:

(5) 2H,0 4(e)- 0 411+ 1.229 v.

Since the oxidation of ferrous ion to ferric ion (reaction 4) proceeds more readily at the anode, it is this reaction which first occurs at the anode at the lowest current density.

As the potential on the cell was raised increasing the current, a change in the relationship of voltage to current occurred at 0.8 v., 0.3 a., indicating a change in the reaction at one of the electrodes. Visual observation of gas liberation indicated that this change occurred at the cathode. The low cathode surface area specifically designed into this cell limited the area of cathode contact with the solution, limited the availability of the ferric ion at the cathode surface for electron transfer. Therefore the next most readily available ion, hydrogen, shared in the electron transfer from the cathode with the subsequent liberation of hydrogen gas (reaction 2).

Because of a high-hydrogen overvoltage on the cathode surface, it is likely that the initial deposit was electrolytic iron (reaction 3). When iron covered the cathode, the hydrogen overvoltage dropped to the normal overvoltage of hydrogen on iron and mostly hydrogen was deposited. As the cell current was further increased, an increasing amount of iron was deposited.

Again at 2.1 v., 5 a., a change in the slope of the V-C cuive began. The polarization was complete at 3.5 v., 10 a., indicating the approximate point at which still another electrochemical reaction was initiated, either at the cathode or at the anode. Upon visual observation of the anode under magnification while 20 a. were being passed through the cell, tiny gas bubbles could be seen evolving from the anode surface. This observation, plus the fact that no further change in the V-C curve occurred at any point up to a cell current of 30 a., leads to the conclusion that the second change in the V-C curve represented the initial liberation of oxygen at the anode.

The current efficiency of the cell in this example was measured at various currents by analyses of the cell effluent to determine the amount of ferrous ion oxidized to ferric ion.

These results are plotted in FIG. 5 of the drawing. With the V-C information and the current efficiency data, one can reasonably specify the current to be used through this cell to obtain the most efficient iron oxidation for the current passed.

Q na

At low cell currents, the desired reaction of Fe (II) Fe (III) at the anode is offset by a high proportion of the reverse reaction, Fe (III) Fe (II), at the cathode. With higher currents, more of the electron transfer at the cathode is handled by hydrogen; hence improved overall efficiency is obtained until at a total cell current of a little more than 6 a., the maximum current efficiency for this cell was approximately 85 percent.

Also, at a. and above, oxygen is liberated, reducing the anode current efficiency and resulting in some direct attack on the graphite anodes.

In addition, as the cell current is increased, iron deposition on the cathode wires increases. The iron must eventually be removed, or the cathode wire size will increase substantially and eventually reduce the efficiency of the cell. The most effective way to remove iron from the cathode was to merely turn the electric current off and allow the iron to dissolve chemically according to the following reactions:

Reaction (6) uses some of the regenerated ferric ion resulting in a reduced overall efficiency. It need not, however, reduce the capacity of the cell at peak loading, because the electrolytically deposited iron can be dissolved at the end of a period or shift of operation or intermittently during a period of operation below the peak demand.

With consideration for current efficiency, minimum iron deposition, and minimum oxygen evolution at the anode, this laboratory cell could be rated at about 6 to 8 a., i.e. an anode current density of from 1.9 amp/dm to 2.5 amp/dm for average loading to be operated intermittently to allow all deposited metallic iron to dissolve, and then operated at 10 a. continuously for several hours to handle peak demands for ferric generation.

EXAMPLE II The small or laboratory cell used in this example consisted of two graphite anode plates, one-fourth inch thick by 2% inches wide, immersed in the electrolyte to a depth of 2% inches and spaced 1% inches apart. Interposed between the anodes were three cathode wires 0.0315 inch in diameter, also immersed to 2% inches depth. Thus the anode-to-cathode area ratio was about 22:1. Between anode and cathode, a reference calomel cell (Beckman 39170 B 84) was immersed and by means of a vacuum tube voltmeter of 20,000 ohm resistance, the voltages between the reference cell and the anode, and between the reference cell and the cathode were measured.

The voltage vs. cell current measurements for this cell in treating a liquor containing 14.65 g./l. Fe (II) and 7.45 g./l. Fe (III), having a pH of 0.7 and a flow rate of 8.5 cm./minute at 60 C. are plotted in FIG. 6

The voltages between the reference electrode and the anode showed that a concentration polarization of the anode began at about 0.2 v., 1.75 a. under the conditions of cell operation. The changing polarization was complete at about 1.3 v., 3.2 a. and a new characteristic slope then continued unchanged. Comparison of this curve with the cell V-C curve led to the conclusion that an electrochemical change at the anode was responsible for the concentration polarization shown on the overall cell voltage curve. Because there are only two oxidizable cations in the solution, and because Fe (II) is known to be more readily oxidized than the hydroxide ion; it can be concluded that a low anode current densities, Fe (II) is oxidized to Fe (111). As the cell current is increased the current density at the anode is increased until a point is reached at which ferrous ions are oxidized to ferric ions as rapidly as they are capable of diffusing to the electrode surface. If the current is further increased at the anode, ferrous ions can no longer reach the electrode surface rapidly enough to accept all of the electrons. A state of concentration polarization exists and some other electrode process must begin-i.e. the oxidation of the hydroxide ion with the discharge of oxygen gas.

The voltage between the reference electrode and the cathode is a straight line function, (FIG. 6) indicating that the electrochemical reactions which are initiated at the cathode upon passage of a low current continue unchanged as the cell current is increased. Because there are three possible cathode reactions, and because there is other evidence that all three reactions do occur, it must be assumed that they all initiate at very low cell currents; and that up to 5.0 a. through this cell, the same reactions continue. The evidence that reduction of Fe (III) occurs at the cathode is based on the fact that the cell current efficiency for oxidation of Fe (II) is less than percent. Metallic iron deposition was measured gravimetrically and evidence of hydrogen evolution was visual.

The concentration polarizations one would expect to see at the cathode are probably there, but are not evident because of the extremely high cathode current density (low cathode area). For example, in this cell, 0.25 a. corresponds to 4 amperes per sq. dm., a current density sufficiently high to have already exceeded the current densities where concentration polarization begins for Fe (III) reduction, metallic iron deposition, and hydrogen evolution. Therefore it is evident that all three reactions will proceed together at rates which are determined by the availability of Fe (III), Fe (II), and hydrogen ion at the cathode surface. Such availability is influenced by the concentration of each of these ions in the solution, the mobility of each of these ions, and the influence of temperature and solution movement (agitation) on the diffusion processes.

In summary, the data of FIG. 6 shows that a concentration polarization on the voltage-current curve is evidence of a change in the anode reaction, rather than any change at the cathode.

EXAMPLE lll Employing the same cell as in Example II, the effect of a change in flow rate of electrolyte on initial anode polarization was determined employing a liquor containing 23.0 g./l. of Fe (II), 5.1 g./l. of Fe (III) and having a pH of 0.55 and a temperature of 60 C. The results of these tests are plotted in FIG. 7 of the drawing. It will be noted that the point of initial polarization increased with increase flow rate up to about 35 cm./minute. Further increase in flow rate had no appreciable effect on anode polarization.

EXAMPLE IV The electrolytic cell employed in this example was a prototype of a full scale commercial cell. It consisted of two graphite plate anodes, 1 inch thick by 14 inches wide. immersed in the electrolyte to a depth of 39 inches and spaced 2 inches apart. Centered between adjacent anodes was a cathode consisting of a 1 inch diameter U-shaped steel frame, 56 inches high and 14 inches between the legs. The frame held 48 horizontal tantalum wires, 0.0315 inch diameter, spaced three-fourths inch apart below the surface of the electrolyte and mechanically held and electrically contacted the wires. All surfaces of the cathode were coated with approximately one-eighth inch thick polyvinyl chloride polymer coating, commercially referred to as vinyl plastisol, except 1 inch at the top of each leg of the frame and 12 inches on each cathode wire. The anode-to-cathode area ratio was therefore about 18:1 so that the cathode current density was always 18 times the anode current density. One hundred sixty-eight gallons of pickling liquor was recirculated through the cell continuously at 12 gallons per minute for an equivalent flow of about 3 feet per minute, and contained 23.6 g./l. of ferrous ion and 3.4 g./l. of ferric ion. The pH was 0.85; the temperature 65 C. One hundred seventy-five amperes were passed through the cell until 640 ampere-hours had accumulated. At the completion of the run, the ferric ion concentration was 5.4 g./l. for a cell current efficiency of 96 percent.

EXAMPLE v Repetition of the above prototype operation starting with 24.3 g./l. of ferrous ion, 5.5 g./l. of ferric ion at a pH of 0.5, a cell current of 150 a. and a total of 580 ampere-hours, produced 1.63 g./l. of ferric ion for a cell current efficiency of 86 percent. The current was applied through a time switching device which applied current for 1 minute, turned the current off for 1 minute and repeated the current on-current off cycle to the end of the operation when 580 ampere-hours were recorded on the ampere-hour meter. No iron was present on the cathode 2 minutes after the current was turned off at the end of the run.

A suitable full scale cell for commercial operation comprises 90 anodes and 84 cathodes set in a plastic lined steel tank 40 inches wide X feet-6inches long by 48 inches deep arranged so that there are anodes and 14 cathodes across the width forming 6 cell subassemblies lengthwise of the tank. Suitable anodes are 1 inch thick by 14 inches wide by 44 /2 inches long and suitable cathodes consist of a three-fourths inch diameter U-shaped steel frame, 56 inches high and 17 inches between the legs with forty-three 0.0625 inch diameter alloy 20 (nickel, chromium, molybdenum, copper) stainless steel wires spot welded to the frame in a horizontal position, equally spaced over 3 feet of the lower area of the frame so that they can be immersed below the 40 inch depth of solution. The resultant anode-to-cathode area is about 10:1.

A cell of this design can be used to regenerate pickling liquor circulated from a 1,550 gallon spray ferric sulfate pickling solution at the rate of 50 gallons per minute through the cell and returning all of the cell effluent to the main tank.

The cell can suitably be supplied with 15,000 a. DC with variable voltage control up to 9 v. for a maximum anode current density of amperes per sq. ft. With an average flow rate past the anodes of 1 foot per minute and with maximum power, the anodes will not polarize and discharge oxygen with normal amounts of ferrous ion present.

The pickling bath is operated within the following limits:

Range Preferred Ferrous ion 20-35 g.ll. 35 g./l. Ferric ions 3-7 g./l. 4 g./l. pH 0.4-0.6 0.5 Temperature 135-l50 F. 140 F.

To maintain the ferric ion concentration a photoelectric detector can be used to signal the passage of a basket of ware entering the pickle tank and the signal from this detector used to start the DC power supply and a timer. After the required elapsed time set on the timer, the current to the cell is automatically shut off. Thus each load of ware initiates its own regeneration of ferric ion. If desired, further control is possible within reasonable limits by manually or automatically controlling the amplitude of current to the cells based on chemical analyses of the pickling bath.

The solution entering the first bank or cell subassembly is at the concentrations of the body of the solution in pickle tank. As it is electrolyzed, at an anode current density of 2.5 amperes per square diameter by the first bank, the ferric ion can be increased about 0.2 g./l. Each succeeding bank also adds 0.2 g./l. of ferric ion so that the solution returned to the tank is richer in ferric ion by about 1.2 g./1. than when it entered.

As each ampere-hour of direct current will oxidize about 1.8 g. of iron, 15,000 a. is capable of oxidizing 27,000 grams per hour (59 lbs. per hour). The etching solution uses about 1.6 grams of ferric ion per sq. ft. of work processed so that 17,000 sq. ft. of work can be processed per hour at peak loadmg.

The electrical cost of operation based on 1 /l(Wl-l and an average loading of about 10,000 sq. ft. per hour, a cell efficiency of about 85 percent, and an electrical conversion efficiency of about 85 percent, the cost of operating the cell is about $8,750 annually. A similar oxidation using hydrogen peroxide costs in excess of$ 100,000 annually.

While there has been shown and described certain specific embodiments and examples of the present invention, it is intended by the appended claims to cover all modifications falling within the spirit and scope of the invention.

What 1 claim as new and desire to secure by Letters Patent of the United States is:

1. An electrolytic process for continually regenerating a ferric sulfate pickling liquor, which comprises:

continually circulating said pickling liquor from a pickling bath through an undivided electrolytic cell which has an anode-to-cathode surface area ratio of from about 6:1 to 22:1 while passing a direct current through said cell at an anode current density of at least 0.5 amperes per sq. dm. to oxidize ferrous ions in said liquor to ferric ions, and returning the effluent liquor from said cell to said bath; said current density not exceeding that at which polarization leading to oxygen discharge occurs at the anode. 2. The process of claim 1 in which the ferrous ion concentration in the liquor introduced into said cell is from 10 to 40 gms/liter and the ferric ion concentration of said liquor is electrolytically increased from 0.5 to 4.0 gms/liter.

3. The process of claim 2 in which the ferric ion content of the liquor introduced into said cell is from 1 to 10 gms/liter.

4. The process of claim 1 in which the anode-to-cathode surface area ratio is from 6: l to 22: l.

5. The process of claim 1 in which the flow rate of liquor through said cell is from 5 to 50 cm. per minute.

6. The process of claim 1 in which said cathodes are composed of metal wire.

7. The process of claim 1 in which the flow of current through said cell is periodically interrupted to dissolve iron plated on the cell cathode.

8. An electrolytic process for continuously regenerating an iron pickling liquor containing ferrous sulfate to oxidize fer rous ions, which comprises:

continuously circulating pickling liquor from a pickling bath through an undivided electrolytic cell having an anodeto-cathode area ratio of from about 6:1 to 22:1 and containing a plurality of parallel graphite plate anodes spaced apart a distance of from 1.5 to 10 cm. and wire cathodes positioned between adjacent anodes and passing a direct current through said cell at an anode current density of from 0.5 to 5.0 amperes per sq. dm. but not exceeding that current density at which polarization leading to oxygen discharge occurs at the anode at the flow rate of said liquor through said cell. 9. The process of claim 8 in which said liquor in said bath contains from 10 to 40 gm/liter of ferrous ions, from 1 to 10 gm/liter ferric ions and has a pH of from 0.25 to 1.5

10. The process of claim 9 in which the ferric ion concentration in said liquor is increased by 0.5 to 4.0 gins/liter while in said cell.

11. The method of operating a ferric sulfate pickling bath consisting essentially of an aqueous solution containing both ferrous and ferric ions and a pH of from about 0.25 to 1.5 and in which the pickling process includes the reduction of ferric ions to ferrous ions, said method comprising:

maintaining the pH of said solution by the periodic addition of sulfuric acid to said bath and the volume of said solution by the per periodic addition of water; and

maintaining the ferric ion concentration of said bath by continually circulating said solution through an electrolytic cell having an anode-to-cathode area ratio of from 6:1 to 22:1 while passing a direct current through said cell at an anode current density of at least 0.5 amperes per square decimeter, but not exceeding that current density at which gas is liberated at the cell anode, to oxidize some of the ferrous ion content of said solution to ferric ions.

12. The method of claim 11 in which the ferric ion concentration of said bath is maintained at from 1 to 10 gms/liter.

13. The method of claim 12 in which the ferrous ion concentration of said bath is from 10 to 40 gms/liter.

1 l l 2 14. The method of claim 13 in which the content flow parallel graphite anodes and wire cathodes and having an through said cell is periodically interrupted for solution of anode to cathode area ratio of from 6:1 to 22:] while a metallic iron plated on said cathode in the solution thereafter passing a di current h h id ll at an anode returned to a bathrent density of from 2 to 3 amperes per square decimeter The method of operallng acld ferric Sulfate Picklmg 5 to oxidize some of the ferrous ion content of said solution bath consisting essentially of an aqueous solution containing to ferric ions from to 40 gm/liter ferrous ions and l to 10 gm/liter ferric The method of claim in which the flow rate of Said ions and a pH of from about 0.4 to 1.5 and in which the picksolution thmugh said cell is from 5 to 50 cm/minute.

ling process includes the reduction of ferric [0115 to ferrous The method of claim 16 in which the ferric ion content ions, said method comprising: 10

maintaining the pH of said solution by the periodic addition zg soluuon 15 Increased about to 4 gms/hter m isald of sulfuric acid to said bath and maintaining the volume of said solution by the periodic addition of water; and The method of clam] 15 m winch Said anodes are FP from 1.5 to 10 cm. apart and said cathodes are positioned maintaining the ferric ion concentration of said bath above l gm/liter by continually circulating said solution through 15 between anodes an undivided electrolytic cell containing a plurality of 

2. The process of claim 1 in which the ferrous ion concentration in the liquor introduced into said cell is from 10 to 40 gms/liter and the ferric ion concentration of said liquor is electrolytically increased from 0.5 to 4.0 gms/liter.
 3. The process of claim 2 in which the ferric ion content of the liquor introduced into said cell is from 1 to 10 gms/liter.
 4. The process of claim 1 in which the anode-to-cathode surface area ratio is from 6:1 to 22:1.
 5. The process of claim 1 in which the flow rate of liquor through said cell is from 5 to 50 cm. per minute.
 6. The process of claim 1 in which said cathodes are composed of metal wire.
 7. The process of claim 1 in which the flow of current through said cell is periodically interrupted to dissolve iron plated on the cell cathode.
 8. An electrolytic process for continuously regenerating an iron pickling liquor containing ferrous sulfate to oxidize ferrous ions, which comprises: continuously circulating pickling liquor from a pickling bath through an undivided electrolytic cell having an anode-to-cathode area ratio of from about 6:1 to 22:1 and containing a plurality of parallel graphite plate anodes spaced apart a distance of from 1.5 to 10 cm. and wire cathodes positioned between adjacent anodes and passing a direct current through said cell at an anode current density of from 0.5 to 5.0 amperes per sq. dm. but not exceeding that current density at which polarization leading to oxygen discharge occurs at the anode at the flow rate of said liquor through said cell.
 9. The process of claim 8 in which said liquor in said bath contains from 10 to 40 gm/liter of ferrous ions, from 1 to 10 gm/liter ferric ions and has a pH of from 0.25 to 1.5.
 10. The process of claim 9 in which the ferric ion concentration in said liquor is increased by 0.5 to 4.0 gms/liter while in said cell.
 11. The method of operating a ferric sulfate pickling bath consisting essentially of an aqueous solution containing both ferrous and ferric ions and a pH of from about 0.25 to 1.5 and in which the pickling process includes the reduction of ferric ions to ferrous ions, said method comprising: maintaining the pH of said solution by the periodic addition of sulfuric acid to said bath and the volume of said solution by the per periodic addition of water; and maintaining the ferric ion concentration of said bath by continually circulating said solution through an electrolytic cell having an anode-to-cathode area ratio of from 6:1 to 22:1 while passing a direct current through said cell at an anode current density of at least 0.5 amperes per square decimeter, but not exceeding that current density at which gas is liberated at the cell anode, to oxidize some of the ferrous ion content of said solution to ferric ions.
 12. The method of claim 11 in which the ferric ion concentration of said bath is maintained at from 1 to 10 gms/liter.
 13. The method of claim 12 in which the ferrous ion concentration of said bath is from 10 to 40 gms/liter.
 14. The method of claim 13 in which the current flow through said cell is periodically interrupted for solution of metallic iron plated on said cathode in the solution thereafter returned to said bath.
 15. The method of operating an acid ferric sulfate pickling bath consisting essentially of an aqueous solution containing from 10 to 40 gm/liter ferrous ions and 1 to 10 gm/liter ferric ions and a pH of from about 0.4 to 1.5 and in which the pickling process includes the reduction of ferric ions to ferrous ions, said method comprising: maintaining the pH of said solution by the periodic addition of sulfuric acid to said bath and maintaining the volume of said solution by the periodic addition of water; and maintaining the ferric ion concentration of said bath above 1 gm/liter by continually circulating said solution through an undivided electrolytic cell containing a plurality of parallel graphite anodes and wire cathodes and having an anode to cathode area ratio of from 6:1 to 22:1 while passing a direct current through said cell at an anode current density of from 2 to 3 amperes per square decimeter to oxidize some of the ferrous ion content of said solution to ferric ions.
 16. The method of claim 15 in which the flow rate of said solution through said cell is from 5 to 50 cm/minute.
 17. The method of claim 16 in which the ferric ion content of said solution is increased about 1.5 to 4 gms/liter in said cell.
 18. The method of claim 15 in which said anodes are spaced from 1.5 to 10 cm. apart and said cathodes are positioned between said anodes. 